1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, it relates to a semiconductor device employing a BPSG film as an insulating film in a very-large-scale integrated circuit (VLSI).
2. Description of the Prior Art
FIG. 2 is a sectional view showing the structure of a conventional semiconductor device which employs a thermal oxidation film of silicon and a silicon oxide film (PSG film) containing phosphorus (P) as insulating films and an aluminum alloy film containing silicon as an interconnection metal film. Referring to FIG. 2, description is now made of the metal electrode interconnection structure of the conventional semiconductor device. First, as shown in FIG. 2, a silicon oxide film 3 and a PSG film 8 are formed on the surface of a silicon substrate 1, and thereafter a contact hole 7 is selectively formed through photolithography or etching. Then an impurity diffusion layer 2 is formed in the vicinity of the surface of the silicon substrate 1 through ion implantation or thermal diffusion. Finally an aluminum alloy film 6 containing silicon is formed through sputtering, chemical vapor deposition (CVD) or the like, to be sintered by heat treatment.
In such a conventional semiconductor device employing the PSG film 8 as an insulating film, it is necessary to increase the temperature for heat treatment or the content of phosphorus to effectively perform flattening with an increase in the aspect ratio (ratio of film thickness to patterning pitch) of the circuit pattern following refinement of the element as shown in FIG. 4. However, the increase in temperature exerts a bad influence on impurity distribution in the element while the increase in the content of phosphorus leads to degradation in moisture resistance. Therefore, a BPSG film (silicon oxide film containing boron B and phosphorus P) is now being watched as a second material for the insulating film since the same shows lower viscosity at the general treating temperature. However, when a BPSG film 4 (FIG. 3) is employed as an insulating film for the conventional semiconductor device as shown in FIG. 2, boron contained in the BPSG film 4 is diffused in the vicinity of the surface of the silicon substrate 1 at the bottom of the contact hole 7 or in the aluminum alloy film 6 containing silicon, to promote movement of the silicon in the aluminum alloy film 6. Thus, a large amount of silicon 9 is precipitated by solid-phase epitaxial growth in the interface between the silicon substrate 1 and an electrode after heat treatment, to increase contact resistance by about one or two figures. Such a problem is remarkable particularly when the content of boron in the BPSG film 4 exceeds 1 percent by weight.
As the prior art of interest to the present invention, "The Use of Titanium-Based Contact Barrier Layers in Silicon Technology", Thin Solid films, 96(1982) pp. 327-345, Electronics and Optics, C. Y. Ting and M. Wittmer, IBM. T. J. Watson R & D discloses that titanium alloy is suitable as a barrier metal for preventing reaction between aluminum and silicon. In this prior art, however, there is no disclosure about employment of a BPSG film as an insulating film for a semiconductor device and structure for preventing precipitation of silicon caused upon employment of the BPSG film. Thus, this prior art is not suggestive of the present invention, although the same employs a barrier metal similarly to the present invention.